This invention relates to a method for controlling an attachment at an agricultural tractor.
Machines for dragging implements, such as tractors, because of their conditions of use and the functions to be performed, and also because of economic considerations are usually constructed without springs. Such vehicles are therefore, very easily and strongly caused to vibrate by unevenness of the roadway because of the elastic properties and low damping of their tires, with the frequencies of excitation usually lying in the area of their natural frequency. The movements of the tractor thus effected by the vibrations lead to considerable disturbances in the operation of an attachment connected to the tractor via a power lift, i.e., an attachment that does not have its own drive elements, such as wheels or the like. Thus, plowing as the most important ground preparation method, with a rigid coupling between tractor and plow, would only be possible with an idealized, completely flat field. In reality, however, the tractor according to its oscillation behavior follows the roughnesses of the field and can thereby be subject to considerable angular deflections, i.e., pitching motions, so that the relative position between tractor and plow must be changed continuously to ensure that the plow remains in the ground.
To this end, for a long period of time, a method based on traction control has been used, which shall be illustrated at this point in connection with FIGS. 1 and 2.
FIG. 1 shows a tractor 1 with a plow 2 which is connected to the tractor via a tie-rod mechanism 3. The tie-rod mechanism is subject to a power lift 4 for varying the relative position between the tractor and the plow, the power lift being actuated by a hydraulic cylinder 5. The cylinder is supplied with pressure fluid via a valve 6 which is controlled by a controller or regulator 7, as will be explained in detail in connection with the block diagram of FIG. 2 illustrating the control circuit. In FIG. 2 the function or transient or step response of the respective control circuit member is illustrated, i.e., the response or starting function respectively, for a sudden steplike change in the input signal. In FIG. 2 t represents time plotted on the abscissa In the conventional traction control circuit the sum of the forces acting in the measuring direction at a measuring spring 8 arranged between a fixed point at the tractor and the tie-rod mechanism 3, is used as the control standard or controlled variable. In the example illustrated the control and measuring direction is parallel to the ground 9. The sum of these forces is thus determined from the traction force, resulting from the depth to be plowed and the corresponding ground resistance characteristic, plus a corresponding component in the measuring direction of the weight of the plow (zero when driving horizontally); with all forces taken into consideration only the respective components in the measuring direction being of importance. The control operates, in this case in such a manner that it detects a change in the force acting on the measuring spring and is controlled to be zero by the movement of the plow.
For the mathematical description in the control circuit, the position of the plow must be determined as to its coordinates, as illustrated in FIG. 1. The coordinate z describes the absolute position of the tractor, whereas the coordinate a describes the relative position between the plow and the tractor. Upon a variation of one or both of these coordinates the absolute position of the plow is changed accordingly, as described by the coordinate a-z, i.e., the value a-z represents the deviation of the plow from its original position Because of a change in the absolute position a-z of the plow and/or because of a change in the characteristic of the ground resistance, which is indicated in FIG. 2 by several characteristic curves in the transfer function in the block diagram for the ground 9, a change in the force F at the measuring spring 8 occurs. The characteristic curves of the ground 9 are intended to encompass changes in the angle of ground inclination or slope or changes in the resistance at the same plowing depth The change in the force F causes a deflection f of the measuring spring, from which the controller forms together with a suitable nominal or set value w an output y which acts on the valve 6 and causes a corresponding pressure fluid flow g per time unit to the cylinder. Due to a lift h of the cylinders following from this fluid flow, the power lift 4 is actuated and the plow is displaced relatively, perpendicular to the ground, by an amount a. In this connection it should be pointed out that the relative movement as well as the disturbance movement of the plow transferred from the tractor result from rotational movements, as for instance the pitch oscillations or the rotation of the tie-rod mechanism about its joints; however, only the vertical components of this movement are of significance, as is represented by the coordinates. This displacement of the plow by the power lift is superimposed with the movement of the tractor z so that a new absolute position a-z of the plow results which in its turn causes a change in the force F as a consequence of the ground characteristic. The control circuit is thus closed.
This control method, as a consequence of a suitable selected constant nominal or set value leads to a constant traction force at the measuring spring, which is achieved via a different penetration depth of the plow and thereby depth of the furrows. With a traction force control therefore a uniform depth of the furrows can be expected only if the changes in the traction force, i.e., the changes in the deformation of the measuring spring were zero. This would require a homogeneous soil i.e., no changes in the resistance with a uniform plowing depth, as well as a path without changes in slope. Furthermore, because of the change of the depth of the furrows via the control a decrease in the revolutions of the motor is prevented, which depending on conditions may lead to the team getting stuck. Such a deadlock can be prevented however only as long as the traction force transmitted by the drive wheels of the tractor can be supported by the ground. If the transferable traction force falls below the nominal value of the traction force due to a deterioration of the frictional force conditions the control circuit calls for an increase in the traction force and to this end reacts in the sense of a positive feedback, i.e., the plow will be lowered further in order to accomplish a higher traction force; consequently slippage of the wheel will increase and this will finally lead the team to get stuck, if no manual correction takes place. A manual interference must be continuously taken also if a uniform depth of the furrows is required which because of the principle of the design of the control circuit cannot be given automatically with the ground and field conditions encountered in reality. As will follow from FIG. 2 the resistance of the ground as the main quantity influencing the traction force to be regulated, forms an element of the control circuit which has a major effect on the dynamics and the sensitivity of the same.
In addition to the traction force control circuit the power lift may also be provided with a position control circuit which facilitates the coupling and decoupling of the attachments and which with implements carried by the tractor, such as fertilizer spreaders, and field spraying apparatus controls the position relative to the tractor. Under special circumstances during working of the field both the traction force and position control circuit may be utilized.
For purposes of realizing this known method in a technical sense with respect to the implements, essentially three possibilities are known:
A mechanical-hydraulic control or closed loop control (MHC) in which the control variables traction force and relative position tractor/plow are mechanically detected and are transferred via a linkage and lever to a slide for the valve of the power lift. This type of control has strict limits regarding stability and sensitivity because of the direct signal feedback without amplification. For instance, with the changes to be expected in the traction force, the measuring spring has to permit deformations which are sufficient for an actuation of the slide, which circumstance, together with the large masses of the attachments leads to signal delays which in addition increase the tendency to oscillations (instability) and decrease the responsiveness of the control circuit. Moreover, this type of (MHC) control, because of friction and play in the mechanical construction of the tie-rod mechanism leads to continuous oscillations which must be suppressed by a wide control resolution or operating threshold which may amount to more than 20% of the maximum traction force.
These disadvantages are reduced by a servo-hydraulic closed loop control in which the control signals are hydraulically conducted and amplified. Play and friction which would favor oscillations do not occur in the hydraulic feedback. In addition the path of deformation required at the measuring spring can be reduced considerably.
Also in an electro-hydraulic closed loop control the principle of the traction force and relative position feedback is maintained while the measurement of the traction force is effected by means of electronic systems. The deformation of a conventional mechanical-hydraulic control (MHC) measuring member is detected by means of an inductive displacement pick-up or transducer which however does not present in principle any functional advantages even though the tie-rod systems and its negative influences on the control are eliminated. With a direct measurement of the traction force in the connection bolts (force measuring bolts) of the control linkage, preferably in the guide-rods, the construction of the power lift is simplified and because of the high stiffness of the measuring member the dynamics of the control system is improved. Just as in the case of the measurement of the forces by means of a potentiometer (potentiometric force measurement) in both guide-rods in a rhombus-like opening (force measuring ring) the disadvantages lie in large loads on the measuring members which may become dangerous because of wear and in the hysteresis behavior caused by mechanical friction, as well as in the arrangement of electronic components such as sensors, in an environment that is hostile to electronic components.
Independently of the choice of the respective realization of the apparatus, control by traction force especially traction force closed loop control because of its control principle has the following disadvantages which can be summarized as follows. In the cultivation of the soil it does not make possible a genuine automation since in practice, especially with ground having a strongly changing resistance characteristic, the depth of the plow must be observed and the control must be manually interfered with. Other methods of operation which require a uniform working plane above the ground can only be executed as a control in which the relative position of the tractor to the plow is the control standard or variable, which because of the tractor pitch movements automatically leads to errors. The structural design of the traction force measuring device poses considerable difficulties since this elastic element must not only measure and support great traction forces but must also transmit the considerable dynamic forces during transport of the heavy attachments. As far as the control technique is concerned the non-linear and non-stationary behavior of the ground as an element of the control circuit is an obstacle in optimizing the traction force closed loop control. Its transfer characteristic enters as a factor into the feedback amplification so that the tendency to oscillations (stability limit) and the response speed as well as the sensitivity and accuracy of the control are determined by the of soil.